1. Field of the Invention
The present invention relates to navigation devices for detecting present positions and for performing positional guidance.
The present application claims priority on Japanese Patent Application No. 2007-247783 and Japanese Patent Application No. 2008-218126, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Conventionally, two types of navigation devices are provided for detecting present positions of vehicles so as to perform positional guidance (or travel guidance) for users, i.e., fixed types which are fixedly installed in vehicles via wirings, and handy or portable types which can be easily handled by users to carry outside and which may be referred to as personal navigation devices (PDN). Recently, fixed types of navigation devices (an example of which is disclosed in Patent Document 1) have been developed such that bearings detected by gyro-sensors or geomagnetic sensors and vehicle information such as running speeds of vehicles (detected by speedometers) are used in addition to GPS signals (where GPS stands for Global Positioning System) so as to detect present locations of vehicles with high precisions. Such functions may be highly evaluated in the case where navigation devices cannot receive GPS signals or in the case where GPS signals do not have high precisions.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H09-42979
It is not always easy for portable navigation devices to receive vehicle information given from instruments on dashboards of vehicles. To cope with the case in which portable types of navigation devices cannot receive GPS signals, auxiliary functions are installed in them so as to detect positions by integrating accelerations detected by acceleration sensors.
Generally, acceleration sensors have offset errors and have serious problems I that they cannot achieve adequately high precision in positional detection. Offset errors may occur when acceleration sensors erroneously detect finite acceleration values irrespective of zero accelerated motion. Offset errors may be output from acceleration sensors which are not placed horizontally in vehicles and acceleration sensors which are installed in vehicles running on slopes inclined from the horizontal plane because of gravitational accelerations, which are erroneously detected as accelerations. Although acceleration sensors are placed horizontally in vehicles, offset errors may inevitably occur in acceleration sensors when vehicles run on slopes.